1. Field of the Invention
This invention relates to beam-addressed light modulators and more specifically to light valve targets comprising dense arrays of electrostatically actuated micromirrors.
2. Description of the Related Art
Beam-addressed light modulators use a scanning electron gun to write a charge pattern onto the pixelized beam-addressing surface of a light valve target. The target imparts a modulation onto a light beam in proportion to the pixel intensities and directs the modulated light beam through projection optics to form an image. Such beam-addressed light valve targets have been demonstrated using transmissive and reflective liquid crystals, reflective membranes and micromirror arrays.
Most of these targets utilize secondary electron emission characteristics to write the charge pattern. As shown in FIG. 1, the addressing surface is characterized by a secondary electron emission curve 2 that plots the emission coefficients .delta., i.e. the ratio of emitted secondaries to incident primaries, against the landing energy of the primary electrons. At landing energies between first and second crossover points (.delta.=1), 4 and 6, the surface exhibits a coefficient greater than one. Outside that region, the surface exhibits a coefficient less than one. In general, clean conductors have coefficients less than one and insulators have coefficients greater than one for useful beam energies.
The electron gun emits primary electrons that strike the target's addressing surface with a landing energy above the first crossover causing more secondary electrons to be ejected than incident primary electrons. The secondaries are collected by a collector grid, which is an open conductive mesh that is preferably spaced about ten times the pixel size away from the addressing surface and aligned to it, so as to not interfere with the scanning electron beam. During writing, the grid is held at a relatively positive potential with respect to the addressing surface to collect the secondary electrons. The charge pattern is written onto the surface by modulating the beam current. During erasure, the grid potential is switched to the anode potential so that the secondaries redeposit themselves on the addressing surface.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays," ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,911, 3,886,310 and 3,896,338. A low energy scanning electron beam deposits a charge pattern directly onto cloverleaf shaped mirrors causing them to be deformed toward a reference grid electrode on the substrate by electrostatic actuation. Erasure is achieved by raising the target voltage to equal the open field mesh, i.e. collector grid, potential while flooding the tube with low energy electrons to simultaneously erase all of the mirrors. This approach increases the modulator's contrast ratio but produces "flicker", which is unacceptable in video applications.
More recently Optron Systems, Inc., as described in Warde et al., U.S. Pat. No. 5,287,215, has developed a membrane light modulation system in which a charge transfer plate (CTP) couples charge from a scanning electron gun under vacuum through to potential wells in atmosphere. A deformable reflecting membrane, which is supported on insulating posts, is electrostatically attracted toward the wells. The CTP serves as a high-density multi-feedthrough vacuum-to-air interface that both decouples the electron beam interaction from the membrane and provides the structural support required to hold off atmospheric pressure.
Warde suggests two ways to write and erase the CTP. The first is very similar to the Westinghouse technique in that the membrane is switched to the collector grid voltage and rescanned to erase the charge pattern, which Warde acknowledges produces image flicker. The second flickerless mode of operation, which Warde refers to as grid-stabilized, applies the video signal to the membrane and fixes the beam current.
Buzak et al, U.S. Pat. Nos. 5,765,717 and 5,884,874 discloses an electron beam addressed liquid crystal light modulator that includes a liquid crystal cell having a target surface, in which a writing electron beam and an erasing electron beam address to provide a display image. The writing and erasing beams sequentially strike preselected locations on the target surface to cause an emission of secondary electrons and, thereby, develop an electrostatic potential at such preselected locations that is applied across the liquid crystal.
A transparent open collector electrode positioned over and above the target surface collects, in a uniform manner, the secondary electrons emitted by all regions of the target surface. A controller circuit sequentially applies first and second potential differences between the target surface and the collector electrode in synchronism with the scanning motion of the writing and erasing beams.
The collector electrode is segmented into four or more electrically isolated segments. As the erase and write guns raster scan the light valve, with the write gun lagging by two segments, the controller switches the potentials on the segments above the erase and write guns to ground and to +300 V with respect to the incident surface. The erase gun secondaries will redeposit themselves over the segment thereby erasing that segment of the charge pattern. The write gun secondaries will be collected by the grid segment thereby writing a new charge pattern. Since both guns operate at energies above the first crossover point, image resolution can be further improved by coating the entire surface of the LCLV with a material such as magnesium oxide (MgO), which exhibits a very high secondary emission ratio, as described in U.S. Pat. No. 4,744,636. The coating acts as a current amplifier, which permits lower beam current for a given charge pattern.
Leard et al., U.S. Pat. No. 5,196,767 discloses an optical signal processor that uses a matrix-addressable field emitter array to supply controlled electron emission to a two-dimensional signal processor element such as a deformable reflective membrane as described in U.S. Pat. No. 4,794,296 or a liquid crystal array. A conductive stabilization grid collects secondary electrons ejected from the signal processor element. The optical signal processor is particularly suited for applications in adaptive optics, optical computing, target recognition, tracking and signal processing and optical communications.